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So I'm going to tell you the etymology instead. Aplomb comes from the French expression, "fil à plomb", which refers to a plumb line, which is usually a string of some sort with a weight attached to the bottom, which is used to provide vertical reference for things like buildings. "Aplomb" came to be an English word taken out of "à plomb", and it means like strength or solidness, referring to the rigidity and sense of equilibrium that a plumb line gives.
